Cursor to selected not working while an armature is applied
I guess your problem is that Blender put the 3D cursor on the original vertex position, i.e. its position without the modifier, it doesn't take the armature modifier into account, which can be annoying sometimes when for example you try to modify a mesh that is affected by an armature: The cursor won't come at the vertex or vertices position, it will stick at the original vertex position, not convenient to grab the XYZ widgets. To make it clear:


Let say you've parented the cube to the bone With Automatic Weight.
Move or rotate the bone in Pose mode.
Select the cube, go in Edit mode. Display the cube's Armature modifier on your right panel. If you enable the Adjust Edit Cage to Modifier Result option, in Edit mode you'll see the mesh fitting to its modified position.
Select a vertex.
Press shiftS > Cursor To Selected.
Contrary to what is expected, the 3D cursor doesn't move to the selected vertex. Actually it sticks to the original vertex position, i.e. its position without modifier. It's Blender's choice, it might have some reasons(?).
If you disable the Adjust Edit Cage to Modifier Result option, you can clearly see what I mean, the 3D cursor stays on the original vertex position.